1. Field of the Invention
This invention relates generally to static inverters and more particularly, it relates to a new and improved pulse-width modulated inverter system and a method for operating the same.
2. Description of the Prior Art
In U.S. Pat. No. 3,538,420 issued on Nov. 3, 1970 to F. N. Klein, there is shown an inverter for providing a A. C. power at a preselectable frequency and a preselectable voltage by generating a waveform having at least one notch in which the widths of the notches are varied to control the output voltage.
In U.S. Pat. No. 3,611,086 issued on Oct. 5, 1971 to B. Mokrytzki, there is disclosed a pulse-width modulated inverter wherein the modulation frequency is an integral ratio of the carrier frequency and such ratio of the carrier frequency is switched automatically during dynamic changes of the motor speed at synchronized times.
In U.S. Pat. No. 3,739,253 issued on June 12, 1973 to A. J. Humphrey et al, there is disclosed a pulse-width modulated power source for providing a waveform between any two of the output load terminals having an initial, middle, and a final voltage pulse in each half cycle thereof. The initial and final voltage pulses are separated by a 120.degree. duration with the middle voltage pulse occupying a section of this 120.degree. duration.
It is generally known that variations in motor speed of A. C. motors can be effected by suitably controlling the frequency of the applied A. C. voltage. However, frequency changes will alter motor torque which is known to be dependent upon the magnitude-to-frequency ratio of the A. C. voltage applied to the motor. In view of this, there have been many prior art attempts to provide power conversion systems which have not only a variable frequency inverter but has also special circuitry such as pulse-width modulation circuitry to regulate the amplitude of the voltage output of the inverter. The problems encountered by these prior art devices were such that pulse-width modulation circuitry required generally the necessity of separate power switching devices together with the accompanied increase of complex control circuits. It would, therefore, be desirable to provide a pulse-width modulated inverter system which can be effected by a minimum number of power switching devices for both frequency and voltage control.
Another problem that existed in these prior art devices which operated over a wide range of frequencies was that they did not take into consideration at higher frequencies a smaller number of pulses should be added for modulating the inverter output due to large power losses and overheating of the power switching devices. Further, it was generally known that as the width of the pulses increased for reducing the output voltage the magnitude of the harmonic content, especially low order harmonics, in such output voltage is increased for a given fundamental voltage thus increasing heating of the motor. Thus, it would be desirable to provide a pulse-width modulated inverter system wherein a minimum number of pulses with the narrowest allowable width are added to the output waveforms at the highest inverter frequency thereby eliminating overheating of the power switching devices. In the present invention as the frequency is decreased the pulse-width is increased continuously for reducing the output voltage until a preselected limit is reached where excess motor heating might be caused by the increased harmonic content. Then, the number of pulses is increased but without changing the total pulse-widths so as to maintain constant the fundamental voltage and yet shifts the harmonics to a higher frequency to decrease motor heating. This process is repeated continuously with additional pulses as the frequency is further reduced.